use PACKED macro insted of open-coding GCC-ism

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

util-linux/volume_id/unused_lvm.c

@@ -22,7 +22,7 @@
 
 struct lvm1_super_block {
 	uint8_t	id[2];
-} __attribute__((packed));
+} PACKED;
 
 struct lvm2_super_block {
 	uint8_t		id[8];
@@ -30,7 +30,7 @@ struct lvm2_super_block {
 	uint32_t	crc_xl;
 	uint32_t	offset_xl;
 	uint8_t		type[8];
-} __attribute__((packed));
+} PACKED;
 
 #define LVM1_SB_OFF			0x400